The present invention relates generally to tire testing of the type that measures tire uniformity and, in particular, to an improved method and apparatus for acquiring more accurate tire uniformity data so that some or all of the irregularities detected in the tire during the testing process can be corrected more precisely.
In the manufacture of tires, various irregularities and variations in the dimensions in the tires can arise. For example, dimensional irregularities can arise from inaccuracies in the molding process, changes in the characteristics of the materials and compounds employed in manufacturing the tires, inaccurate centering and variations in the vulcanization process, etc. All of the possible irregularities and variations in the tires, which can arise during manufacture, either singularly or through interaction with one another, can cause eccentricity, static and dynamic unbalance in the tire, and force variation which can result in tire vibration or noise during use.
It is possible to correct many of these irregularities by first measuring the tire variations and applying various corrective actions to the tire. To measure the variations, the tire is placed in a tire uniformity inspection machine. In currently available tire uniformity inspection machines, testing is fully automatic. Tires are fed by conveyor to a test station where each tire is mounted upon a chuck, inflated to a predetermined pressure and rotatably driven at a standard speed with its tread surface in abutting contact with the circumferential surface of a loadwheel. The loadwheel is instrumented with load cells that measure forces due to the tire acting on the loadwheel. The data gathered during the testing process may be used to grade the tire and/or to take immediate corrective action via shoulder and tread grinders, which selectively grind rubber from regions of the tire to compensate for the variations detected during the testing process. Alternately, or additionally, the data taken during the testing cycle may be used to mark specific regions of the tire to alert the installer to an area of interest, such as an irregularity or point of high force in the tire, which will enable the installer to take corrective or compensating action during the installation of the tire onto a wheel.
In a typical tire testing system the loadwheel is free to rotate about an axis parallel to the rotational axis of the tire. The loadwheel has spindles at its opposite ends provided with load cells which measure forces acting on the loadwheel in directions of interest. Precise measurement of the forces exerted by the tire permits accurate adjustment of the uniformity of the tire after the force measuring procedure, for example, by grinding devices which remove excess tire material to correct any irregularities that may have arisen during the manufacturing process.
An example of a tire testing system and a load wheel construction is described in copending U.S. applications Ser. Nos. 08/988,480 and 08/988,509, respectively. In these systems, the loadwheel spindles are provided with load cells that are secured to a movable carriage. The carriage is attached to a ball screw housed driven by a motor and gear reduction unit. Rotation of the screw shaft moves the ball screw and carriage toward or away from the tire being tested, the carriage sliding along the frame of the machine. A servomechanism moves the carriage to a desired position based on the force signals generated by the load cells.
Although prior art tire testing systems, and in particular known loadwheel assemblies used therewith, measure tire uniformity in an acceptable manner, several drawbacks exist so as to leave room for improvement. The loadwheel assembly (which includes a loadwheel, axle and bearings) is fairly heavy. As a result, the load cells receive and respond to the motion of the overall machine frame, as well as the forces produced by the rotating tire. The motions in the machine frame can be caused by external vibration, such as that produced by the traffic of industrial trucks near the machine or by vibrations internal to the machine, such as that produced by the operation of tire conveyors or grinders that form part of the testing machine.
The forces caused by these extraneous vibrations are undesirable, and represent errors in the measurement system. In prior art devices, attempts have been made to avoid these errors by having the tire testing machines installed on massive foundations or, alternately, sequencing the machines in a way that avoids actuating or moving the subassemblies of the machine (such as grinders) while measurements are being taken. These prior art methods have substantially added to the cost of the installation and have also increased the operating costs due to increased cycle time and reduced throughput of the machine.
The present invention provides an improved method and apparatus for obtaining tire uniformity data of a tire being tested in a tire uniformity machine. In the preferred and illustrated embodiment, the apparatus includes a loadwheel assembly including a rotatable loadwheel. Load sensors or load cells for detecting forces imposed on the loadwheel by a tire being tested also form part of the assembly. A vibration sensor for detecting vibrations in the loadwheel is also provided. The vibrations being monitored are generally caused by movements in the frame of the machine caused by industrial lift truck traffic in the vicinity of the machine or movement of components, such as conveyors and grinders within the machine itself. The signal or information obtained from the vibration sensor is subtracted from the overall signal or data generated by the load cells in order to provide more precise tire uniformity data.
The disclosed apparatus reduces or eliminates the error that sometimes occurs in tire uniformity data measurements due to extraneous forces or vibrations that are received by the load cells during the testing of a tire.
In the preferred embodiment, the vibration sensor comprises an accelerometer mounted to the loadwheel assembly. In the illustrated embodiment, at least two accelerometers are used, one for detecting the lateral force component and one for detecting the radial force component. In a more preferred construction of the illustrated embodiment, two radial accelerometers are used, because due to existing componentry, a single radial accelerometer cannot be mounted in alignment with a radial plane of the loadwheel assembly. As a result, two symmetrically-spaced radial accelerometers are used.
In the illustrated embodiment, the signal generated by an accelerometer is communicated to a differential amplifier via a conditioning circuit and a scaler. The scaling factor used by the scaler is determined by the characteristics of the accelerometer being used. A signal from the load cells which represents the total force on the loadwheel is also communicated to the differential amplifier. The total force includes both forces generated by the tire being tested, as well as the forces generated by vibrations applied to the tire machine. The resuting net signal from the differential amplifier is one that more precisely reflects the actual tire uniformity data of the tire being measured.
Additional features of the invention will become apparent and fuller understanding obtained by reading the following detailed description in connection with the accompanying drawings.